This invention generally relates to conveyor rolls, and is specifically concerned with the use of a flat sided coil spring retainer for non-rotatably mounting end caps to a ceramic roller member.
In glass sheet tempering systems, a conveyor mechanism having a plurality of elongated, ceramic rolls is used to convey glass sheets in and out of the heating chamber of a furnace. Such rolls are generally formed from fused silica whose ends are rotatably driven and supported by the conveyor mechanism. In order to transmit the driving torque from the conveyor mechanism to the ceramic roll, a pair of metallic end caps are non-rotatably mounted on either end of the roller member. In one type of conveyor system, each metallic end cap includes a short stub axle which is concentrically aligned with the axis of rotation of its respective end cap. The stub axles of the end caps are rotatably received within U-shaped members present on the upper ends of a pair of opposing support arms of the conveyor mechanism. A drive belt under one or both of the opposing end caps imparts a drive torque on the roller member which allows it to rollingly convey a sheet of heated glass. In other types of conveyor systems, stub axles may not be present on the end caps, and the end caps may be driven by gear trains or chains.
However, regardless of the type of conveyor system the conveyor roll works in combination with, each of the metallic end caps must be non-rotatably mounted to its respective end of the roller member so that the torque generated by the drive member of the conveyor mechanism is effectively transmitted to the ceramic roller. Any slippage between the end caps and the ends of the ceramic roller member would substantially impair the operation of the conveyor roll in transporting the glass sheets. Hence, there has been a long felt need for a simple, economical and effective means for non-rotatably attaching the torque-transmitting metallic end caps to the ends of the ceramic roller members used in such rolls.
Unfortunately, the thermal environment that such conveyor rolls must operate in, in combination with the thermal and mechanical properties of the materials used in such rolls, has frustrated the development of a truly satisfactory retention means between the end caps and the roller members. When such rolls are subjected to the operating temperatures associated with glass sheet tempering systems (i.e., between 500.degree. and 1000.degree. C.), the end caps may reach temperatures approaching 500.degree. C. or more. The thermal expansion induced in the metallic end caps under such temperatures is much greater than the amount of expansion induced in the cylindrical ends of the roller member which are disposed within such caps. Consequently, a substantial mechanical loosening will occur between the caps and the ends of the roller member if the end caps are merely frictionfitted onto the cylindrical ends of the roller member. If such loosening becomes severe enough, the affected roller may become a "dead roll" which scrapes and damages the surface of the taffy-like, heated glass as it is pushed along the conveyor mechanism by the driving force imparted by the other driven rolls. While it would seem, at first blush, that the retention problem would be solved simply by mechanically interconnecting the end caps to the roller by means of screws, clamps, pins, or other conventional fasteners, the difficulty of machining bores, threads, or other recesses in the brittle silica roller members, in combination with the localized stresses that such mechanical fasteners impose on the silica rollers, disadvantageously results in localized cracking.
Accordingly, a number of alternative approaches have been attempted in the prior art to effectively interconnect the end caps with the rollers in ways which will avoid the creation of crack-inducing stresses on the roller ends. In one such approach, a heat-resistant silicon-based adhesive is applied between the cylindrical outer surface of the roller ends, and the inner surfaces of the tubular walls of such end caps. Unfortunately, the silicon-based adhesive must be periodically replaced in such conveyor rolls, as it breaks down over time. A more promising approach has been the utilization of spring-like members between the end caps and the ends of the roller member that continuously apply a frictional retaining force between these two elements. Such an approach is exemplified in McMaster U.S. Pat. No. 4,404,011 which utilizes a plurality of adjacent leaf spring members, as well as in the Page U.S. Pat. No. 4,399,598, which utilizes circumferentially expandable and radially compressible split metal rings.
While both of these approaches appears to be an advance over the use of presently known silicon-based adhesives, neither of these approaches is without its shortcomings. For example, neither of the retaining approaches disclosed in either the McMaster '011 or Page '598 patents provides any means for positively preventing the end caps from axially sliding off the ends of their respective roller member. This is important, as the axial slippage of such an end cap from a roller during the operation thereof could result in the creation of either a "dead roll", which scrapes the surface of the softened glass, or a roll which rotates eccentrically and causes unwanted, optically distortive variations in the thickness of the glass. Additionally, the split metal rings utilized as the retaining element in the Page '598 patent would not appear to be capable of applying a strong retaining force between the roller member and the inner surface of the tubular wall of the end caps when the end caps had thermally expanded, unless the corrugations in the split rings were dimensioned in the radial direction such that they applied a very high wedging force when the end cap was slid over its respective roller end during the assembly thereof under ambient temperature conditions. While the radially compressive forces would probably relax when the roll became heated during use, the application of such large, radially compressive forces during assembly could apply localized stresses on the ends of the roller which could result in micro cracking or other damage. Additionally, in order to install the roller member in end caps having such split rings in their interiors, it may be necessary to axially force the end caps into the roller, which in turn applies even more potentially stressful forces in the brittle material forming the roller. Finally, the relatively large gap between the free edge of the end caps and the cylindrical body of the ceramic roller member present in the Page '598 design invites the accumulation of dust and debris between the end caps, the split rings and the ceramic roller member when the roller member is in use. Such accumulation could ultimately impose unwanted stresses on the roller ends when the end caps and the metal split rings cooled and contracted over the roller ends when the glass heating furnace was turned off. While the use of eight flat leaf springs as shown in the McMaster '011 patent could well provide a retaining means which avoids some of the aforementioned shortcomings associated with the design of the Page '598 end cap retainer, the fact that the side edges of the leaf springs are not mechanically interconnected could allow these springs to circumferentially shift during the operation of the roll, which in turn could result in a non-concentric retention of the roller end within the tubular wall of the end cap. Such a non-concentric retention could cause the roll to rotate eccentrically. Additionally, the assembly of this particular device is relatively difficult and awkward, as temporary circumferential clamps must be used to squeeze the leaf springs together around the end of the roller member before the end cap can be slid over the springs.
Clearly, there is a need for a retaining means that positively secures the end caps of a conveyor roll to the ends of its respective roller member not only in the circumferential direction, but in the axial direction as well. Ideally, such a retaining means should positively retain the roller to the end caps despite the highest levels of thermal differential expansion, and should further be relatively easy to assembly without the imposition of large mechanical stresses onto the roller ends. Finally, it would be desirable if such a retaining means were relatively inexpensive to manufacture, highly reliable in performance, and durable in operation.